drm/ttm: Add debugfs output entry to pool allocator.

ttm_page_alloc_debugfs can be registered to output the state
of pools.

Debugfs file will output number of pages freed from the pool,
number of pages in pool now and the lowes number of pages in
pool since previous shrink.

Signed-off-by: Pauli Nieminen <suokkos@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>